There is a continous humming sound coming from the boiler (hot water heat). Over a period of 15 to 20 minutes it goes from a low volume hum to a very irritating volume level. Then, something happens that reduces the volume back down to nearly nothing, and the cycle starts over again. I've "listened" to various elements of the system with the old metal-rod trick, but can't pin-point the culprit.

It almost sounds like a pressure build-up, but I can't pin it down. Any information that will help solve this problem will be appreciated.